Field of the Invention
The present invention relates to a prism for a projection optical system included in a projector, and in particular to a prism for a projection optical system suitable for a projector having a laser light source as an illumination light source and an optical system having the same.
Description of the Related Art
Projectors having image display devices such as reflective liquid crystal panels and DMDs (digital micro-mirror devices) have been known as apparatuses for projecting digital information such as video contained in video recorders and documents and images contained in personal computers. Such a projector includes an illumination light source configured to radiate illumination light, an illumination optical system configured to guide illumination light from the illumination light source to image display devices, and a projection optical system configured to guide projection light from the image display devices to a projection lens.
The illumination light emitted from the illumination light source is guided and split into light rays of three colors of red, blue, and green through the illumination optical system, and applied to the image display devices associated with the respective colors. The illumination light rays applied to the image display devices are emitted as projection light rays by the image display devices, and the projection light rays are combined again, guided to the projection lens and projected onto a screen.
In addition, a prism (also referred to as a total internal reflection prism: TIR prism) for a projection optical system is used to totally reflect either one of the illumination light and the projection light and transmit the other. Specifically, the prism for a projection optical system is an optical member used for both of an optical system for guiding illumination light and an optical system for guiding projection light, and includes a combination of multiple triangular prisms. Furthermore, the prisms are combined in a manner that the prisms are opposed with an air gap formed therebetween, and optical paths are set so that surfaces in contact with the air gap totally reflect illumination light and transmit projection light.
For the illumination light source, high-intensity xenon lamps, high-pressure mercury lamps, and the like are used in the related art. In recent years, LEDs (light emitting diodes) and laser light sources using semiconductor lasers have been in practical use with improvement in luminous efficiency and increase in the amount of luminescence. In particular, since blue laser light sources are developed, light sources for illumination light using light sources of three primary colors, which are the blue laser light sources, green laser light sources, and red laser light sources, have been developed.
If light (projection light, for example) passing through a surface in contact with the air gap of the prism for the projection optical system is partly reflected, light leaks into the prism, which not only degrades light use efficiency but may also damage optical components, adhesives, and the like owing to heat caused by unnecessary reflected light. In particular, when a laser light source with high power output is used, the degree of heat due to unnecessary reflective light becomes high, which is undesirable.
Thus, a prism system provided with a pitch periodic structure smaller than the wavelength of incident light on a surface (a total internal reflection surface, for example) in contact with an air gap to suppress reflection and improve light transmission characteristics at the air interface and a projection display device having the same have already been proposed (refer to JP 2004-191878 A, for example). In addition, an image projection system including an illumination light source having laser light sources of three primary colors of blue, green, and red, light combining unit, a total internal reflection prism, and a reflective image display device has already been proposed (refer to JP 2008-111889 A, for example).
In an optical system using a prism for a projection optical system, it is desirable to lower the reflectivity of light emitted to an air gap after passing through the prism and the reflectivity of light (projection light, for example) entering the prism from the air gap to ensure light transmission. If light to be transmitted is partly reflected, the reflected quantity is a loss in the light quantity and the light use efficiency lowers, which is a problem.
In general, since the reflectivity of light incident at a high angle is high, the reflectivity of projection light is desired to be lower in such a configuration as a prism for projection optical system in which projection light enters and exits at relatively high angles.
In particular, when high power output laser light is used as illumination light, even if only part of light to be transmitted is reflected, there are concerns about damages on optical components and adhesives due to heating by unnecessary reflected light and image degradation due to multiple reflection in an air gap because of high power, which is further a problem.
The optical system disclosed in JP 2004-191878 A provides the pitch periodic structure smaller than the wavelength of incident light on the surface in contact with the air gap to suppress reflection and improve the light transmission characteristic at the air interface. To provide such a periodic structure, however, not only a special process is required but also it is difficult to clean dirt or the like, if any, adhered on a surface after production, which is disadvantageous in cost and manufacturing processes.
The optical system disclosed in JP 2008-111889 A prevents unnecessary light from the image display device from entering a dichroic film but applies a split process thereto to improve the contrast of a projected image. Sufficient consideration, however, is not given to suppressing unnecessary reflection when projection light passes through a total internal reflection surface to suppress unnecessary heating and improve light use efficiency.